Optus says "changes to routing information" after a "routine software upgrade" was behind last week's nationwide outage, affecting 10.2 million Australians and impacting 400,000 businesses.

So it turns out that the cause was indeed a rogue change they couldn't roll back as we had been speculating.

Weird that whatever this issue is didn't occur in their test environment before they deployed into Production. I wonder why that is.